1. Field of the Invention
The present invention relates to a printhead and a printing apparatus using the printhead.
2. Description of the Related Art
Recently, digital copying machines and printers are rapidly coming into practical use. In particular, digital color printers and color copying machines are becoming the mainstream in the field of color printers and color copying machines because they can exploit a digitization feature of facilitating color adjustment, image editing, and the like.
Printing methods adopted in these printing apparatuses include an electrophotographic method, inkjet method, and thermal transfer method.
Of inkjet printing methods, in case of using a method of discharging ink droplets by heat generated by an electrothermal transducer (heater), continuous printing raises the printhead temperature. Along with this, the temperature of ink in the printhead also rises. For this reason, the inkjet printing apparatus popularly uses ink whose viscosity decreases as the temperature rises, slightly increasing the amount of ink discharge from the printhead.
A change in amount of discharge greatly influences the printing quality. To correct the changed amount of discharge, a method of changing the heating time of a heater has been widely employed. That is, the amount of ink discharge increases in proportion to the heating time. By using this characteristic, the amount of discharge is controlled to be constant regardless of the temperature change by shortening the heating time by a time corresponding to an increase in amount of discharge upon temperature rise.
The size of recent printheads is becoming large in order to deal with a growing number of nozzles and color printing. In particular, an elongated printhead with a printing width of 4 inches or more suffers great temperature variations in the printhead, and the heating time of a heater needs to be changed for each nozzle. To meet this need, the U.S. Pat. No. 6,116,714 discloses a method of applying, to each nozzle, a mechanism of selecting a proper pulse from a plurality of pulse signals having different heating times. As an input pulse selection method, methods disclosed in Japanese Patent Laid-Open No. 07-101077 and the U.S. Pat. No. 5,969,730 have been known.
FIG. 9 is a diagram showing the arrangement of a conventional printhead control circuit.
In the conventional arrangement, one of heat pulse signals is selected, and the selected pulse waveform is directly used to drive a printing element.
The arrangement and operation of the control circuit shown in FIG. 9 will be explained.
A serial data signal 10 made up of a pulse selection signal and print data for each nozzle is serially input to a shift register 1 in accordance with a data clock signal 11. When a data latch signal 13 is input to a print data latch circuit 2, the print data latch circuit 2 latches, as parallel data signals 12, the print data out of the serial data signals stored in the shift register 1. The data latch signal 13 is also input to a selection information latch circuit 31, and the selection information latch circuit 31 latches the pulse selection signal out of the serial data signals stored in the shift register 1.
A plurality of pulse signals 19 are input to a heat pulse selection circuit 6 via a plurality of signal lines, and the heat pulse selection circuit 6 selects one of the pulse signals 19 in accordance with a pulse selection signal 32 output from the selection information latch circuit 31. The heat pulse selection circuit 6 outputs the selected signal as a heat pulse signal 20 to a NAND gate 7. The print data latch circuit 2 outputs print data 14 to the NAND gate 7.
The NAND gate 7 performs a NAND operation, and outputs a “Low”-level signal when both the print data 14 and heat pulse signal 20 are valid. The output signal is input to a power transistor (driving element) 8. The power transistor 8 is turned on when the output from the NAND gate 7 is at “Low” level. After the power transistor 8 is turned on, an electrothermal transducer (heater) 9 is driven, and an electric current flows into the electrothermal transducer 9 to generate heat. Upon the heat generation, heat energy is supplied to ink, and the ink bubbles and is discharged from a nozzle (not shown).
In FIG. 9, reference numeral 21 denotes a driving power supply for the electrothermal transducer 9; 22, a logic power supply for a logic circuit; and 23, a ground signal.
FIG. 10 is a circuit diagram showing the detailed arrangement of the control circuit shown in FIG. 9. In particular, FIG. 10 is a circuit diagram for one heater when there are four pulse signals 19. The four pulse signals are represented as input pulse signals (0), (1), (2), and (3). Note that one heater corresponds to one nozzle for discharging ink.
FIG. 11 is a timing chart showing timings when the print data latch circuit 2 and selection information latch circuit 31 latch the serial data signal 10 in the circuit shown in FIG. 10.
In the circuit shown in FIG. 10, the serial data signal 10 is sequentially input in the order of nozzle 0, nozzle 1, nozzle 2, . . . , nozzle m, . . . , and nozzle n, as shown in the timing chart of FIG. 11. The input data are print data A0 and pulse selection signals B0 and C0 of two bits for nozzle 0, print data Am and pulse selection signals Bm and Cm of two bits for nozzle m, and print data An and pulse selection signals Bn and Cn of two bits for nozzle n.
In the shift register 1, the serial data signal 10 is shifted in accordance with a data clock signal as shown in the timing chart of FIG. 11. This operation shifts all data for nozzle 0 to nozzle n in the shift register 1, and data corresponding to nozzle 0 to nozzle n are set in the shift register 1. As a result, print data Am of nozzle m shown in FIG. 11 is output to output Q(3m+2) of the shift register 1, and the pulse selection signals Bm and Cm of nozzle m shown in FIG. 11 are respectively output to outputs Q(3m+1) and Q(3m).
In this state, the data latch signal 13 is input. At the leading edge timing of the signal pulse, the print data latch circuit 2 latches the print data A0, . . . , Am, . . . , and An. At the same time, the selection information latch circuit 31 latches the pulse selection signals B0 and C0, . . . , Bm and Cm, . . . , and Bn and Cn.
The selection information latch circuit 31 outputs each latched pulse selection signal as the 2-bit pulse selection signal 32 to the heat pulse selection circuit 6 for each nozzle. The heat pulse selection circuit 6 selects one of the four pulse signals 19 in accordance with the pulse selection signal 32, and outputs the selected signal as the heat pulse signal 20. When both the heat pulse signal 20 and print data 14 are valid, the power transistor 8 is turned on to send an electric current to the electrothermal transducer (heater) 9 and discharge ink.
FIGS. 12A and 12B are timing charts of an input pulse signal and heat pulse signal.
FIG. 12A is a timing chart when the heat pulse selection circuit 6 selects input pulse (2). In this case, the value of the 2-bit pulse selection signal 32 is “10” (e.g., “1” for Bm and “0” for Cm) in binary representation. Input pulse signal (2) is selected and output as the heat pulse signal 20.
FIG. 12B shows all selectable heat pulse signals when four pulse signals (input pulse signals (0) to (3)) are input. As is apparent from FIG. 12B, four types of heat pulse signals can be output for four input pulse signals.
By this control, an optimum heat pulse signal is selected from a plurality of pulse signals for each nozzle, thereby discharging ink.
To apply inkjet printing apparatuses to a field such as the printing industry in which there is a strict requirement for the printing quality, variations in amount of ink discharge must be reduced much more than the conventional ones. For this purpose, the heating time of a heater needs to be controlled to change more finely.
The heater has a protective film to prevent corrosion of the heater by ink. Repetitive discharge shaves and thins the protective film. This improves heat transfer to ink, increasing the amount of ink discharge. In this manner, the amount of ink discharge changes depending on even the time of printhead use.
Particularly in a printing apparatus with a full-line printhead in which the printhead is fixed and printing is performed while conveying a print medium, the amount of discharge greatly differs between nozzles. Depending on the time of printhead use, variations in amount of discharge between nozzles gradually become large, thus deteriorating the printing quality.
To solve this problem, the heating time must be changed for each nozzle to make the amount of discharge equal to that of another nozzle in accordance with the degree of deterioration of the heater protective film that proceeds over time.
For this reason, the number of selectable input pulses needs to be increased to finely control the heater heating time over a wide range.
The conventional arrangement shown in FIG. 10 selects a plurality of selectable input pulse signals, and directly uses the pulse waveform as a heat pulse. Adjusting the amount of ink discharge requires input pulse signals equal in number to necessary heat pulses. For example, when the number of necessary heat pulses is 16, the number of necessary input pulse signals is also 16. The U.S. Pat. No. 6,116,714 described above also discloses a method of selecting two input pulse signals and combining the two pulse waveforms to generate a new heat pulse.
FIGS. 8A and 8B are timing charts for explaining generation of a heat pulse signal according to this method.
To increase the number of combinations of heat pulse signals, a complicated design task is indispensable for waveform shaping of input pulses and combination of input pulses. If even one necessary heat pulse signal changes, the waveforms and combinations of input pulses must be redesigned from the beginning. Even these conventional methods directly use and combine the pulse waveforms of input pulse signals, and do not finely control the heater heating time over a wide range.
However, to increase the number of input pulse signals in an actual printhead and printing apparatus, the numbers of circuits and terminals must be greatly increased. This means upsizing of a printhead and printing apparatus and the rise of cost, and low apparatus cost and low running cost, which are advantages of inkjet printing, cannot be attained.